In its basic form, an electronic camera, such as digital still camera or a camera embedded in a mobile phone (cellphone or smartphone) or in a tablet computer includes two components: (1) a lens module comprising a set of one or more plastic or glass lens elements and used to create an optical image of the viewed scene, and (2) an image sensor (e.g. CMOS or CCD), which converts the optical image to the electronic domain, where the image can be processed and stored. There are different types of electronic (or digital) cameras ranging by application (e.g., regular SLR, camera-phone, automotive, security and medical) and by functionality (e.g., with or without auto-focus). The simplest cameras are those in which the lens module is fixed in position with respect to the image sensor. These cameras are often called fixed-focus cameras, where the camera is focused to a pre-determined distance. Objects that fall within the depth of field (DOF) of the camera can be captured sharply and those which fall beyond the DOF will be blurred. In more advanced cameras, the position of the lens module (or at least one lens element in the lens module) can be changed by means of an actuator and the focus distance can be changed in accordance with the captured object or scene. In these cameras it is possible to capture objects from a very short distance (e.g., 10 cm) to infinity. Some advanced camera designs may include different groups of lenses that can move with respect to each other and hence change the effective focal length of the camera, which results in optical zoom capability. The trend in digital still cameras is to increase the zooming capabilities (e.g. to 5×, 10× or more) and, in cellphone cameras, to decrease the pixel size and increase the pixel count. These trends result in greater sensitivity to hand-shake or in a need for longer exposure time. This has created a need for optical image stabilization (OIS), which now appears in advanced cameras. In OIS-enabled cameras, the lens or camera module can change its lateral position or tilt angle in a fast manner to cancel the hand-shake during the image capture.
In compact (miniature) camera modules, the most ubiquitous form of an actuator is the Voice-Coil Motor (VCM), which includes a coil (wire turned on a cylinder), fixed (or “permanent” or “hard”) magnets and springs. When current is driven through the coil, an electro-magnetic (EM) force is applied and the lens module changes position. While the VCM is considered a mature technology, it is costly, large in size, uses rare-earth magnets, is slow in operation and consumes high power.
Co-owned PCT patent application PCT/IB2014/062836 teaches a different type of actuator that essentially replaces the permanent magnet(s) of a VCM with one or more ferromagnetic members. Such an actuator is termed herein a “magnetic reluctance actuator” (MRA). The MRA disclosed in PCT/IB2014/062836 is “stable”. In a stable actuator, if the actuator is moved from a position of magnetic/mechanic force equilibrium and then released, without a change in the coil current, it will return to the same equilibrium point. In an unstable MRA under the same condition, the actuator will return to a completely different point (usually a start or end point). This is caused by the strong change of magnetic force with position (gap size).